The present disclosure generally concerns optical filters, and more particularly, an optical wavelength band-blocking filter using birefringence and polarization wavelength discrimination techniques.
Optical wavelength filters pass or attenuate light as a function of wavelength of light energy in the signal to be filtered. Various practical applications may benefit from a filter that can selectively tune to a particular wavelength, such as modulation/demodulation devices, imaging apparatus, spectral analyzers, and other devices and techniques.
A filter that passes a selected wavelength band with a high transmission ratio, and attenuates other bands, is considered a band-pass filter. Tuning a band-pass filter to pass a given wavelength band while attenuating energy in unselected wavelength bands is functionally similar to tuning the filter so as to block the unselected bands, except that simply tuning a bandpass filter to a wavelength other than a selected bandpass wavelength does not achieve the result needed in a band stop filter, namely to eliminate the stop band with a high rejection ratio while allowing other bands to be transmitted. The usual filter application involves selecting for a narrow pass band and attenuating the remainder of the spectrum. In the present disclosure, one object is to eliminate a narrow band, termed the “stop band,” while preserving transmission in other bands insofar as possible. Eliminating a selected stop band may be useful, for example, to block undesired wavelengths to better enable the reception of other wavelengths.
In an interference type periodic wavelength filter, bands of low attenuation (passbands) and bands of high attenuation (stop bands) alternate at regularly spaced wavelengths due the operation of the filter. If the object is high discrimination of a particular wavelength, a narrow passband or stop band is an asset. Another useful aspect is a wide free spectral range between discriminated bands. A filter parameter known as finesse represents the ratio of the discriminated band width (full band width at half maximum amplitude) to the free spectral range (wavelength span between discriminated bands).
It would be ideal in a band pass filter to have no attenuation in the discriminated pass band and 100% attenuation otherwise. By extension, a stop band filter would ideally have 100% attenuation in the discriminated band and pass the rest of the spectrum without attenuation. This poses some challenges in an interference filter if the physical processes that produce a narrow discrimination band also result in poor free spectral range, and the processes that increase the free spectral range also undesirably increase the width of the discriminated band.
Parent U.S. Pat. No. 6,992,809 discloses a pass band filter wherein a cascade of filter stages includes one or more stages having a transmission spectrum with narrow passbands and one or more other stages having large free spectral range. By cascading, the rejection ratios of the stages are superimposed and multiplied. The passbands that are widely spaced operate to select among the passbands that are narrow but closely spaced. The cascaded filter finesse is the product of the finesse attributes of the stages.
A technique wherein the discrimination bands of one cascaded stage select among the discrimination bands in another cascaded stage may be useful in a band pass application but it may not be possible to achieve a similar benefit in a band stop filter. For example, if a broadband signal is attenuated by a stop band at a given stage, e.g., adjacent to an input, then any overlapping stop band in a later stage cannot narrow the overall stop band. The later stage might broaden the stop band or it might increase the rejection ratio at a particular wavelength, but cannot contribute to improved finesse. This demonstrates an inherent distinction between pass band and stop band filters.
In the case of a band stop filter, an arrangement of cascaded stages, with respectively narrow and widely spaced discrimination bands, would be characterized by a broad stop band and low free spectral range, i.e., the opposite of what is desirable in a high performance filter. Therefore, what is needed is a way to provide multiple stages that contribute a progressively greater distinguishing difference in a physical attribute that can be used as a filter selection criteria, using a robust distinguishing technique.
Two characteristics may be important in the band stop filter, namely, the ability to discriminate between nearby wavelengths, preferably so as to permit selection of a narrow band, and also a high rejection ratio or difference in transmission/attenuation between the pass band and the any other band(s).
What is needed is another way to process a signal to render the stop band clearly distinct from the remainder of the spectrum, in a way that permits an ultimate selection apparatus to separate the stop band exclusively. In a high performance application, another important need is a very high rejection ratio. In an application where the wavelengths to be rejected are high in power level, a robust arrangement is needed that is not prone to overheat.